Mine dropped right onto the Aero Precision lower, no issues - had to be that old-skool Oly lower that was doing it. I've got a 2003 Oly lower (and upper) that I have to tap the takedown pin into, once the pivot pin is locked. That same upper drops right down on other lowers I've got. It's definitely something that they do with their lowers, and I swear I read somewhere that they do it for a tight, "no-slop" fit... PITA sometimes... Sights and optics... Since it's a .45 ACP round, and it's not going to win any long distance competitions, I figured mine was good for 150 yards, though I've stretched it out to 200 yards. I used a mini red dot setup from Primary Arms, in the American Defense QD mount, with MagPul flip-ups. <thumbsup> It's a thought... <dontknow> <laughs>